Extreme ultraviolet lithography (EUV lithography, also known as soft x-ray projection lithography) is a contender to replace deep ultraviolet lithography for the manufacture of 0.13 micron, and smaller, minimum feature size semiconductor devices.
However, extreme ultraviolet light, which is generally in the 5 to 40 nanometer wavelength range, is strongly absorbed in virtually all materials. For that reason, extreme ultraviolet systems work by reflection rather than by transmission of light. Through the use of a series of mirrors, or lens elements, and a reflective element, or mask blank, coated with a non-reflective absorber mask pattern, the patterned actinic light is reflected onto a resist-coated semiconductor wafer.
The lens elements and mask blanks of extreme ultraviolet lithography systems are coated with reflective multilayer coatings of materials such as molybdenum and silicon. Reflection values of approximately 65% per lens element, or mask blank, have been obtained by using substrates that are coated with multilayer coatings that strongly reflect light essentially at a single wavelength within an extremely narrow ultraviolet bandpass; e.g., 12 to 14 nanometer bandpass for 13 nanometer ultraviolet light.
This process requires costly glass with extremely low thermal expansion and mask blanks require months to fabricate. The glass needed for extreme ultraviolet lithography needs to be thin, extremely smooth, extremely sharp, and free of defects. Any imperfections to the lens element or masks can cause problems in the final product.
For example, because of the nature of the multilayer stack and small feature size, any imperfections in the layers will be magnified in the final product. Imperfections on the scale of a few nanometers can show up as printable defects on the finished mask and need to be eliminated from the surface of the mask blank before deposition of the multilayer stack. Thus, cost and time saving solutions need to be found while still maintaining the precisions and quality required for and extreme ultraviolet lithography.
In view of the need for the increasingly smaller feature size of electronic components, it is increasingly critical that answers be found to these problems. In view of the ever-increasing commercial competitive pressures, along with growing consumer expectations, it is critical that answers be found for these problems. Additionally, the need to reduce costs, improve efficiencies and performance, and meet competitive pressures adds an even greater urgency to the critical necessity for finding answers to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.